The current mainstream in the semiconductor industry resides in diodes, transistors, ICs, LSIs and VLSIs of the resin encapsulation type. Epoxy resin compositions comprising an epoxy resin, curing agent and additives have superior moldability, adhesion, electrical properties, mechanical properties, and moisture resistance to other thermosetting resins. It is thus a common practice to encapsulate semiconductor devices with epoxy resin compositions. Recently, the semiconductor devices are increasing their degree of integration and accordingly becomes larger in chip size. By contrast, the outer configuration of packages is reduced in size or thickness in order to meet the demand for size and weight reduction of electronic equipment. As to the method of mounting semiconductor parts on circuit boards, the surface mounting of semiconductor parts becomes predominant since the parts on boards are increased in density.
For the surface mounting of semiconductor devices, a method of dipping entire semiconductor devices in a solder bath and a method of passing them through a hot zone where solder is melted are commonly employed. These methods bring about thermal shocks, by which the encapsulating resin layer can be cracked or separation occur at the interface between the lead frame or chip and the encapsulating resin. Such cracks or separation becomes more outstanding if the encapsulating resin layer on semiconductor devices has picked up moisture prior to the thermal shocks during surface mounting. In actual working processes, it is impossible to avoid the moisture absorption of the encapsulating resin layer. As a consequence, the semiconductor devices encapsulated with epoxy resins often suffer from a serious loss of reliability after surface mounting. One common countermeasure against such a popcorn phenomenon is to load epoxy resins with large amounts of filler for reducing moisture absorption. Also one approach for improving the molding of thin packages is to reduce the viscosity of epoxy resin compositions. Further, fast curing catalysts have also been studied to shorten the molding cycle for increasing productivity.
Prior art curing catalysts, for example, imidazole derivatives, tertiary amine compounds, tertiary phosphine compounds and derivatives thereof are less stable during shelf storage. When resins are milled therewith, these curing catalysts give rise to such problems as an increased viscosity upon milling and poor flow upon molding.
JP-B 56-45491 discloses an epoxy resin composition comprising an epoxy resin and a curing agent. The curing agent is obtained by heat treating a mixture of a novolac type phenolic resin and tetraphenylphosphonium tetraphenylborate (abbreviated as TPP-K, hereinafter) at a temperature above the softening point of the novolac type phenolic resin until the resin system is colored to be yellowish brown or brown. Allegedly this epoxy resin composition is shelf stable and produces cured products having improved moisture resistance. However, because of a low activity and poor fast-curing ability, this curing catalyst must be used in large amounts, rather detracting from the shelf stability of the epoxy resin composition.
TPP-K itself is a useful curing catalyst. Although it has good latency in that reaction starts above a certain temperature, it lacks fast-curing ability. JP-A 9-328535 discloses a thermosetting resin composition using as a catalyst a modified compound of TPP-K with only the boron atom being substituted. However, this modified compound of TPP-K with only the boron atom being substituted has a catalytic activity which cannot fully comply with the recent demand for fast curing. Thus phosphorus catalysts having higher activity are desired. Triphenylphosphine which is generally used from the past has a superior fast-curing ability, but is poor in shelf stability.
The prior art curing catalysts are thus difficult to produce epoxy resin compositions which are improved in shelf stability and flow and have excellent properties including fast curing, latency and moisture resistance.